A U.S. helicopter exchanged fire with Pakistani troops near the border between Afghanistan and Pakistan today, resulting in injuries for two Pakistani soldiers, U.S. and Pakistani officials said.

Accounts differ as to what exactly prompted the firefight, but the incident may further strain already fragile relations between the U.S. and Pakistan following the American unilateral military raid deep in Pakistani territory that killed Osama bin Laden May 1.

The U.S.-led International Security Assistance Force said U.S. helicopters were in Afghanistan near Forward Operating Base Tillman when they responded to incoming direct and indirect fire from over the border in Pakistan, presumably from militants. The helicopters initially did not return fire, but when a second round of incoming fire began, they did fire in response.

The Pakistani military, however, said that two NATO helicopters caused the incident by violating Pakistani airspace before being fired upon by Pakistani troops.

"We know for sure the [Apache] helicopter was fired upon -- we got rounds inside the helicopter," said Gen. John Campbell, East region commander for the ISAF. "The helicopter returned fire and we are working through just exactly what happened... If [American soldiers] are taking effective fire, then by all means they have to take all measures to safeguard themselves and the other people around there."

The ISAF said it is investigating the incident further, but one senior U.S. official said it is "likely" the helicopter accidentally did pass into Pakistan. Campbell told ABC News that insurgents sometimes fire on NATO troops from near Pakistani outposts in hopes of drawing U.S. return fire.

Though the Pakistani military lodged a "strong protest" over the incident, one Pakistani military official said the army made it clear it was not overly concerned when it requested the meeting at the colonel or brigadier general level. Had the incident been fatal or had the Pakistani military wanted to object more strongly, it would have made the request to meet at a higher level or raised stronger complaints at a diplomatic level, he said.

U.S. Commander: Lack of Communication With Pakistan Dangerous

Campbell said that communication between the American and Pakistani militaries had suffered in the backlash following the bin Laden operation, which has increased chances of such incidents on the border.

"You just have to be talking back and forth ... so if something comes from Pakistan and somebody has fired, we can pull up our Pakistani counterparts and say, 'Hey, we are getting fire from here. We need you guys to go take care of that,'" he said. "So it's very important to try to work that relationship."

Gen. David Rodriguez, the ISAF commander in Afghanistan, said it is "very tense along the border" and that American troops are "trying to be as careful as we can, as have the Pakistanis."

The U.S. raid in Abbottabad, Pakistan, that killed bin Laden earlier this month, which was apparently carried out without the full knowledge of Pakistani officials, provoked an angry backlash from the Pakistani public and the Pakistani government. Last week the Pakistani parliament passed a resolution that "condemned the U.S. unilateral action in Abbottabad, which constitutes a violation of Pakistan's sovereignty."

Pakistan Protests After Bin Laden Raid

Sen. John Kerry, D-Mass., chairman of the Senate's Committee on Foreign Relations, recently returned from a trip to Pakistan in which he held meetings with several senior Pakistani government and military officials meant to help the two countries "push the reset button."

Back in Washington, D.C., today, Kerry chaired a lengthy meeting of the Committee on Foreign Relations dedicated to U.S.-Pakistan relations in which several committee members expressed their frustration in dealings with Pakistan.

Retired Gen. James Jones, Jr., former National Security Advisor to the White House, appeared before the committee and said America could improve relations in the aftermath of the bin Laden raid, despite the struggle.

"It is a difficult moment," he said, "but it is a moment of opportunity if cooler heads prevail."

In perhaps the first sign of trust-building to come out of Kerry's visit, Pakistan agreed Monday to return the pieces of a secret U.S. helicopter abandoned in the Navy SEAL bin Laden operation.